Mercury 40-Ampere Stator and Capacitor Discharge Ignition

Electrical and electronic topics for small boats
Polarue
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Joined: Fri Aug 31, 2018 3:22 pm

Mercury 40-Ampere Stator and Capacitor Discharge Ignition

Postby Polarue » Fri Aug 31, 2018 3:46 pm

Hello everyone, I have been researching information on the 40-Ampere stator charging system that is on my 1998 Mercury 175-HP 2.5-liter v6 EFI two-stroke outboard that I recently purchased. I have spent much time on this engine, and I want to share some additional information regarding this engine and many similar Mercury engines in hopes of helping others who find themselves in the situation of wondering exactly how this system is made and functions.

The stator is often blamed for electrical problems--and they are not cheap. You want to make sure that a replacement stator is absolutely required before installing one. To let you know where my information comes from, it comes from a complete disassembly of the stator, pick-up coil, flywheel, and switchbox, both OEM and one aftermarket. I have enough electrical knowledge to reverse engineer the switchbox, so I think that I can probably shed some light on the mystery of the Mercury capacitor discharge ignition (CDI) system for others. I have upgraded and rebuilt my switchboxes, and hope they never fail again.

If anyone has any questions for me, I will share what I know.

Has anyone else ever rebuilt their switchbox?

Polarue
Posts: 11
Joined: Fri Aug 31, 2018 3:22 pm

Re: Basics of the Mercury 40-Ampere Stator

Postby Polarue » Fri Aug 31, 2018 4:49 pm

Basics of the Mercury 40-Ampere Stator

There are two main coil groups.

First, the large coils that are connected to two pairs of yellow wires provide for a total of close to 40-Amperes output at high engine RPM. The 20-Amperes plus 20-Amperes coil pairs are rectified from alternating current (AC) to direct current (DC) and regulated down to 14-Volts in the two rectifier-regulators mounted at the rear.

The other group of four coils, two red and two blue, are exclusively for the switchbox and spark ignition. One each of the red and blue coils are each fed to their respective switchbox. In the switchbox, there is a blocking diode that allows voltage to flow from the red wire to the blue wire, but not the other way. This allows the red or "high-speed" coils to assist in charging the capacitor, even at low speed if the voltage on the red wire is higher than on the blue (or "low-speed" coil) and higher than on the capacitor at that moment. More on that later.

The alternating current power pulses in the red ("high speed") and blue ("low speed") coils are rectified by a half-wave rectifier, which leaves power for the other half-wave unused. This design creates less heat in the coil. The high speed (red) coils have fewer windings and use larger-gauge wire, therefore they produce less voltage per revolution but higher current than the blue-wire ("low-speed") coils at low engine speed.

Polarue
Posts: 11
Joined: Fri Aug 31, 2018 3:22 pm

Re: Basics of the Mercury Switchbox Circuit

Postby Polarue » Fri Aug 31, 2018 5:11 pm

Basics of the Mercury Switchbox Circuit

The power enters the switchbox via the blue and red wires [from the associated coils]. This alternating-current power is then rectified on the positive half-wave only; the negative charge pulses are not used. This rectified half-wave of positive voltage is sent directly to charge up the approx 1.5-μFarad 630-Volt capacitor that fires the spark.

The flywheel has six magnets; each switchbox fires three times per revolution. Thus two magnets charge up the blue and red coils between firing cycles. This means two voltage pulses from each coil per revolution are sent to the capacitor. However, the only way for the red coil ("high-speed") to contribute to the charging of the capacitor is if the voltage on the blue wire ("low-speed" coil output) and the voltage on the capacitor are both lower at that time. Once the first magnet passes the blue and red coils for that switchbox, the voltage in the capacitor is higher than the peak voltage in the red wire at low RPM. So if the capacitor is not fully charged after the first magnet passed, the second magnet passing the coils will only produce high enough voltage in the blue coils("low-speed") due to their higher number of turns to complete the charge in the capacitor.

Once the SCR releases the charge in the capacitor to the the ignition coils, the capacitor charge empties, the SCR [stops conducting] and the cycle repeats.

jimh
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Location: Michigan, Lower Peninsula
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Re: Mercury 40amp stator design plus ignition

Postby jimh » Fri Aug 31, 2018 5:18 pm

Great information. Please continue.

Polarue
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Joined: Fri Aug 31, 2018 3:22 pm

Re: How the Trigger Circuit Works

Postby Polarue » Fri Aug 31, 2018 5:30 pm

How the Trigger Circuit Works

The trigger assembly consist of three coils spaced 120-degrees apart around the hub magnet in the flywheel. The flywheel hub magnet is two poles, one North and one South.

As a South pole passes a coil, a positive pulse is in one wire from the coil. The other wire is positive when the North pole passes. This is how the two pole magnet trigger three cylinders, each via the three coils. Each pole triggers one switchbox.

If one switchbox is receiving lower voltage pulses than the other, the hub magnet could have lower flux on one of the poles. The magnet material in the hub magnet is like the soft flexible fridge magnet material, not like the hard outer magnets.

If the hub magnet develops more than one set of magnetic poles, as could occur from cracks in the magnet material, the switchboxes can start to fire at the wrong time or misfire.
Last edited by Polarue on Fri Aug 31, 2018 6:55 pm, edited 1 time in total.

Polarue
Posts: 11
Joined: Fri Aug 31, 2018 3:22 pm

Re: SCR circuit

Postby Polarue » Fri Aug 31, 2018 5:44 pm

The SCR Circuit

Each trigger wire actually produces two pulses per revolution of the flywheel which are rectified by the switchbox. Only the positive pulse is used to trigger a SCR which in turn triggers a second larger SCR that releases the capacitor energy to the ignition coil. The SCR only needs 1.5-Volts (maximum) to trigger and allow maximum current flow. The two SCRs are isolated in the circuit via a small coil-transformer. When the capacitor is empty, the SCR automatically [stops conducting].

Polarue
Posts: 11
Joined: Fri Aug 31, 2018 3:22 pm

Re: Mercury 40amp stator design plus ignition

Postby Polarue » Fri Aug 31, 2018 5:56 pm

Side note on CDI efficiency.

Due to the use of capacitors, the energy loss is 50-percent in the ignition circuit just from the capacitor alone. A much more efficient ignition would be to have the main 12-Volt battery drive the ignition coils and use the SCR switching to turn them off and on. These 140-Volt coils would not be useable, but the automotive coil-on-plug packs would do nicely.

The above ignition would eliminate the high-low coils altogether and lead to a lot fewer stator failures.

Polarue
Posts: 11
Joined: Fri Aug 31, 2018 3:22 pm

Re: Cost to Rebuild Switchboxes

Postby Polarue » Fri Aug 31, 2018 5:59 pm

Cost to Rebuild Switchboxes

On another side note, It cost me $20 to rebuild and upgrade my four switchboxes to bulletproof, but the stator is another project.

Main differences in the quality of the OEM compared to the aftermarket switchbox that I own are:

  • Higher quality board, posts, nuts and thicker 2 layer copper on oem
  • Name brand components on OEM
  • Neither box used heat sinking for the large current SCR
  • OEM sealing and encasing dielectric compound is much tougher to strip than aftermarket
  • Cost to upgrade components at the manufacturing level is less than $1 per box, however much larger losses at the dealership level due to loss of revenue on parts and repairs

My parts were small lots with shipping costs and I still have extras.

I can provide a greater level of detail if anyone is interested.

Polarue
Posts: 11
Joined: Fri Aug 31, 2018 3:22 pm

Re: The Bias Circuit in Switchbox

Postby Polarue » Tue Sep 04, 2018 12:29 pm

The Bias Circuit in Switchbox

Another aspect of the switchbox function is the BIAS circuit.

The BIAS circuit is provided on a wire with white insulation with a black stripe that connects the two boxes. This circuit has a negative voltage when read with a voltmeter. The manual states -1 to -6 Volts at low engine RPM, but my experience is that the aftermarket boxes can read up to -8 Volts at an engine speed of 400-RPM (as during engine cranking). These are all negative voltages.

The bias circuit is necessary for a couple of reasons. First, the SCR (Silicon-controlled Rectifier) only requires 1.5-Volts maximum to go into conduction. As the rotational speed of the center hub magnet increases to 400-RPM from 0-RPM at cranking, the trigger coils will provide a signal of 4-Volts to the boxes. This needs to be attenuated down as it is more than 1.5-Volt and ensures a reliable trigger signal. As the RPM continues to rise, the voltage of the trigger pulses continues to rise as well. For racing applications, the trigger signal voltage requires more of the voltage to be reduced at top engine speeds, and that is why the bias resistance is less at 9-kilo-Ohms for racing engines compared to 14.3-kilo-Ohm for fishing engines.

Secondly, the trigger coils rotate under the flywheel and the fit is loose. The strength of the magnetic field from the hub magnet varies with the distance so if the trigger assembly is not perfectly centered, each trigger coil will produce different voltages. My trigger was so loose that I had a 0.7-Volt difference in trigger coil voltages at cranking. To remedy this situation, I used epoxy to build up the outer surface of the trigger coils until it was a snug fit inside the stator. I sanded the inside stator steel surface smooth and greased the surface so the epoxy would wear well. I managed to reduce the voltage variance down to 0.15-Volts across all three trigger coils. The bias circuit will try to equalize the trigger signals, but it is best to start with a balanced and even trigger voltage from a centered trigger assembly. As the trigger advances the timing by rotating, it now stays centered.

The boxes (powerpacks) have to have their BIAS circuits connected because the average motor may not have the exact same voltages from all three trigger coils feeding the switch boxes. Different voltages pulses from the trigger will shift the timing of the sparks if they are not biased. The idle switch is open in neutral and closes (connects to ground) when in gear. This switch connects a 6,800-Ohm resistor (white with black stripe) from the bias circuits in the switch boxes to ground, and this should advance the timing approximately 3-degrees. If you want more advance, you could lower the resistance in that wire. The bias circuit is internal to the boxes. Gounding the bias terminal directly to ground would be harmful as it would advance the timing too much. Also, it may take out your switch boxes.

Polarue
Posts: 11
Joined: Fri Aug 31, 2018 3:22 pm

Re: Stator Construction

Postby Polarue » Sat Sep 08, 2018 11:22 am

Stator Construction

More on the stator and its construction.

The 40-Ampere stator uses ten coils of 14-AWG magnet wire for the 14-Volt alternating current energy generated.

The smaller four ignition coils use 35-AWG wire for the high-speed coils and 41-AWG for low-speed coils.

The blue wire (low-speed) coil failed on my stator. On the coil that failed, the nylon spool (bobbin) that forms the coil melted and burned. A poor solder connection to the terminal appears to have failed; the melting was not due to coil overheating. This was was a quality control problem, not a design defect. I will say that 41-AWG wire is hair-thin and working with this product is difficult. The low speed coil has 3,000-feet of wire spooled onto it and only weighs in with one ounce of copper.

The red high speed coils have even less copper--half an ounce of copper--but since the wire size is 35-AWG, there are only 330-feet spooled.

I rewound my own coils with 38-AWG (blue or low-speed) and 33-AWG (red or high-speed) single build, 200-°C magnet wire. The original wire for the low speed coils was quadruple-build magnet wire. Those layers take up space and coatings are cheaper than copper. My coils generate less heat losses and weigh over three-times more than the originals.

The coils also had to be larger. With the wire I used to achieve the same turns, it took approximately 4,500-feet of wire per coil for the low-speed (blue) coils and 500-feet of wire per coil for the high-speed (red) coils . I could not physically count the original number of turns. But I could measure the weight, diameter, and resistance of my coils to identify the necessary length required, and from that I could know how much wire in the size I chose was required for the same number of turns on the original spools.

Magnet wire is standardized and you can download a chart with all the necessary information to spool your own.

jimh
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Re: Mercury 40amp stator design plus ignition

Postby jimh » Wed Sep 19, 2018 7:36 am

Thank you for the very interesting information. I had heard about "high speed" and "low speed" stators used on Mercury outboard engines, but had no real understanding of how they worked.

Can you share any photographs of your rebuilt coils?

Can you give us a sketch of the alternator charging system and the capacitor discharge ignition system?